The nucleoporin Nup98 gene has recently emerged as a frequent target of chromosomal rearrangements in acute myelogenous leukemia (AML). Nup98 gene rearrangements result in the expression of chimeric proteins consisting of the N-terminal portion of Nup98 fused to 1 of at least 15 different proteins. In most cases, the fusion partner is a transcription factor of the homeobox family. The best-characterized Nup98 chimera is Nup98-HOXA9, which contains the N-terminal portion of Nup98 and the DNA-binding domain of the homeobox transcription factor HOXA9. Nup98-HOXA9 transforms cells in vitro and induces AML in mouse models. However, its direct target genes and the mechanisms by which it causes leukemia are not known. Our preliminary data suggests that Nup98-HOXA9 acts as a transcription factor with a stronger and wider transcriptional activity than wild-type HQXA9, in contrast to other leukemia-associated transcription factors, such as AML1-ETO or PML-RARa, that act as dominant negative suppressors of their wild-type counterparts. In addition, we have shown that Nup98-HOXA9 expands primitive human hematopoietic progenitors in vitro. The studies in this proposal will test the hypothesis that Nup98-HOXA9 is an aberrant transcription factor, identify its mechanisms of action, and elucidate the mechanisms by which it causes leukemia. We will a) determine the effect of Nup98-HOXA9 on proliferation, stem cell numbers, differentiation, apoptosis, and cell cycle in primary human CD34+ hematopoietic stem/progenitor cells and identify the genes that mediate these effects; b) demonstrate that Nup98-HOXA9 is a DNA-binding transcription factor and identify its direct target genes and cognate binding sites; c) identify Nup98-HOXA9-interacting proteins and their effect on its functions; d) elucidate the basis for the increased transcriptional activity of Nup98-HOXA9 compared to wild-type HOXA9; and e) identify cases of AML with Nup98 gene rearrangements using a FISH break-apart probe, identify abnormal gene expression patterns in these cases, and correlate the findings with those obtained from in vitro-transformed primary human CD34+ hematopoietic stem/progenitor cells. Taken together, these studies will clarify the mechanisms by which Nup98-HOXA9 contributes to leukemogenesis and identify critical target genes, promoter binding sites, and protein interactions that could serve as potential targets for therapy.